.beta.-mercaptopropionic acid is a suitable crosslinking agent for acrylic ester polymer, a suitable hardening agent for epoxy resin, and is used in synthetic resins for lenses, etc.
U.S. Pat. No. 5,256,818 to Tomioka discloses a method for making .beta.-mercaptopropionic acid (HSCH.sub.2 XH.sub.2 COOH) by reacting alkaline thiodipropionate obtained from solid thiodipropionic acid, with alkaline sulfide. Specifically, sodium or potassium hydroxide is used to convert solid thiodipropionic acid to the corresponding thiodipropionate, which in turn is reacted with sodium sulfide at a temperature of 110.degree. to 130.degree. C. for one or several hours, preferably in the presence of residual alkaline hydroxide. The resulting solution is acidified with a strong acid to form the .beta.-mercaptopropionic acid.
In an alternative embodiment disclosed in U.S. Pat. No. '818, the starting material is acrylonitrile, which is reacted with an excess of alkaline hydrosulfide or sulfide. Alkaline hydroxide is added, and alkaline .beta.-mercaptopropionate is formed. Acidification with a strong acid yields .beta.-mercaptopropionic acid, with alkaline sulfate is a by-product.
However, each of these methods requires relatively rigorous reaction conditions to convert the thiodipropionic acid to 3-mercaptopropionate. Such conditions can create problems with the composition of the product as .well as the undesirable formation of by-products.
It therefore would be highly desirable to prepare 3-mercaptopropionic acid more efficiently, at lower reaction temperatures and with shorter reaction times.